Both topoisomerases (I & II) have been purified to homogeneity from mouse leukemia L1210 cells and used to study drug effects in purified systems. Topoisomerase II inhibition by DNA intercalators and epipodophyllotoxins was studied by determining the enzyme- induced cleavage labeled SV40 DNA. Each class of drug enhanced selective enzyme-induced DNA cleavage sites. However, DNA sequence analysis at the cleavage sites did not reveal any consistent analogy. The effects of DNA groove binders upon topoisomerase II inhibition by anticancer drugs were analyzed. Both the cellular polyamines, spermine and spermidine and the antibiotic, distamycin reversed drug-induced enzyme inhibition. This effect was associated with decreased total DNA cleavage and enhanced cleavage at selective sequences. We propose that polyamines redistribute topoisomerase II along the DNA and increase enzyme concentration at DNA sequences where the enzyme is not sensitive to topoisomerase II inhibitors. Camptothecin had been reported to inhibit mammalian topoisomerase I. We have studied this inhibition further by comparing 21 camptothecin analogs and by analyzing the DNA sequence selectivity of the cleavage sites in SV40 DNA. A good correlation was found between topoisomerase I inhibition and antileukemic activity, which suggests that topoisomerase I is the target of the anticancer action of camptothecins. Structure activity relationships of the analogs indicate that camptothecins interact with a specific enzyme-DNA receptor. Finally a consensus DNA sequence was identified at the camptothecin-induced DNA cleavage sites.